Since the beginning of the 1970's there have been attempts to produce electrochemical structural elements such as accumulators or the like in the form of thin layers. The goal has been to obtain composite films that are both flexible enough that they can be, for instance, rolled up or made to conform to another desired shape and that also have particularly favorable charging and discharging properties due to an extremely high contact area between the individual electrochemical components, such as electrodes and electrolytes, relative to the volume of the active electrochemical material used. Apart from a few exceptions, this construction (film technology) was to present substantial advantages. In general, it will not be used only if (a) extreme requirements are to be made on the system, or (b) particular electrochemical advantages exist.
In patent literature, a number of methods for producing such films has been described. As far as films are concerned that are provided as electrolyte layers in structural elements to be produced therefrom, two different approaches exist.
According to the first approach, a paste is produced from all essential components. Said paste then serves as a basic material for the film. For producing the paste, a solid electrolyte is dissolved in the paste material; thereupon, a wetting or a cross-linking thin layer of said electrolyte is deposited within the film by extraction of the solvent. In the U.S. Pat. No. 5,009,970, polyethylene oxide (PEO) is used as a polymer which is mixed with an adequate lithium salt in water, whereby a complex of the two components is obtained. The PEO is cross-linked by radiation. A hydrogel is obtained that is to be dried subsequently under vacuum. U.S. Pat. No. 5,041,346 also describes an oxymethylene cross-linked variant of an ethylene oxide polymer electrolyte which additionally contains a softener. However, it has been reported that although the ion conductivity of such complexes compared to pure solid lithium salt is drastically increased, it is still not sufficient for use as an electrolyte layer in electrochemical structural elements. In fact, the most homogeneous deposition is obtained by said method; however, the high price of a mechanical instability of the film obtained (tearing, rolling up, sticking) has to be paid. A further disadvantage is that the soluble lithium conductors that are used according to this technology are hygroscopic, partly even susceptible to hydrolysis. Moreover, water is not only adsorbed, but usually incorporated as crystal water. Besides a very complicated storage of the films produced in this manner (storage has to be made in drying chambers), the films can practically not be laminated without steam bubbles, since the water, due to the tight bond to said substances, is not extractable by conventional methods. Decontactings, holes to the point of small explosion craters and a deliquescence of the laminate are usual results, for which reason said method is advantageously only applicable to pastes only.
According to the second strategy, a microporous sponge structure is provided. In this respect, U.S. Pat. No. 5,456,000 describes rechargeable battery cells which are produced from self-supporting films by lamination of electrode and electrolyte cells. A film or a membrane is used as positive electrode which has separately been produced from LiMn2O4 powder in a matrix solution of a copolymer and has subsequently been dried. The negative electrode consists of a dried coating of a powderized carbon dispersion in a matrix solution of a copolymer. Between the electrode layers, an electrolyte/separator membrane is provided. For this purpose, a poly(vinylidene fluoride) hexafluoropropylene copolymer in acetone or THF or the like is reacted with a plasticizer that is suitable as a solvent for electrolyte salts. The films produced from these components are laminated. For activating the battery, it is immersed into the respective electrolyte solution, thereby soaking with the electrolyte solution.
Due to the high proportion of plasticizer, the films show a very bad aging resistance; after storage periods of several weeks, modifications in consistency and brittleness to the point of decomposition to powder are observed which is possibly due to an interaction with environmental moisture. Moreover, due to the high proportion of plasticizer, lamination can only be effected at a temperature which is considerably decreased relative to the melting point of the polymer. Therefore in the variant described as preferred, the plasticizer is expelled in advance, which requires expensive washing steps. Moreover, the absorption capacity of the electrolyte is reduced, since a large proportion of the pores is reduced in size or even closed by laminating the washed films. Therefore, it is particularly preferable to wash the cell after lamination only. The washing step yet causes tensions and decontactings in a cell produced with said film; the mechanical stability is thus considerably affected. Also, electrochemical decompositions are observed if the cell is activated in a later stage. A further disadvantage is the direct contact of the subsequently filled-in liquid electrolyte with the contact gauzes which are usually made from aluminum on the positive side and from copper on the negative side, said direct contact being due to the porous structure. Consequently, decompositions of the electrolyte between two metals without reference can occur.
Neither is it advantageous therefore to incorporate the electrolyte homogeneously into the organic paste material provided for the production of the films, as has been proposed so far, nor is it advantageous to provide a high degree of porosity of the films that has to be provided by washing out plasticizer—usually several times.
The problem of the present invention is to provide films having improved properties from which electrochemical structural elements, particularly accumulators and batteries, can be produced in the form of thin composite layers. The films thus produced shall not present the aforementioned disadvantages of prior art. Moreover, paste-like masses are provided from which such films can be produced.